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The
LOUJ-OhTn
JJleter
ALL RESISTORS :t~%
Did you eyer try to get a good resistance
read ing on a speaker voice coil?
Or an automobile ignition balla st resistor? '
Or a pilot light bu lb , length of coax, relay
contact , loading coil, identi fy ing transformer
windings, centertaps, etc .? If you
have, then yo u' ve found that most ohmmet
ers don' t have a rea lly low resista nc e
scale - or if they do the zero set ting is
unreliable and the meter scale inad equ at e.
Aft er years of annoyan ce from this problem
I finally decided that a low-value
ohmmeter wou ld be a handy device to have
around. In fact , applications for this meter
just seem to suggest themselves. Why
bother building a new version of an old
instrument and not inc or porate new innovati
ons? So I came up with the se requirements:
• No zero ing knob - just a " set it
and forget it" calibration cont rol.
• Linear scale read ing from zero to a
fixed value, rather than to inf inity.
What good is infinity to an ohmmete
r, besides making the upper
25% or so of the scale relatively
useless?
• Solid terminal connectors rath er
than wires with probes (optional).
This would prevent poor contacts
in the circui t, leading to error.
The finished version uses one size D
flashlight cell, has t wo scales - zero to
lOQ, and zero to I Q . A " test" switch
(momentary pushb utton) is used because
of the high current requirem ent of the
circuit. Battery life is conserved in this
way.
Theory of operation
The circuit consists of a constant-
FEBRUARY 1971
A. Sch ecner W3YZ C
122 She rry Lane Apts.
Conshoho cken, PA 1942 8
curr ent gen erator (Q 1) Lo provide a known
Current through the test resistance. The
voltage drop across this resistance (100 mV
maximum) is read on a meter whose
internal resistance is much higher than a
test resistance of 10Q. Therefore , the
met er does not load t he circuit.
A bias for Ql is established by diodes
Dl and D2 and resistors RI, R2 , R3, R4 ,
and R5 . Use of the diod es hold s the bia s
level constant desp ite nominal de cay of
battery voltage. Diode D3 pr events severe
overload of the meter in the absence of a
test resistance. In case the test button is
pres sed, the meter will overload, but not
much, thanks to D3.
My met er was "Obtained when I scrapped
a Heath grid-dipper...... after bu ying a solidsta
te model. The meter requiremen ts are
500 IlA full scale, 200Q resista nce. A
substitute may be constructed by using a
mo re sensitive meter with a shun t, so long
as the result is 500 IlA, 200n.
Construction
Layout is not critic al, but sold er connect
ions must be solid and medium bus
wire must be used. The battery connector
mu st make tight connection with the batte
ry . Check inte rn al resistance by shorting
the test terminals wit h heavy wire. Pressing
the test button should give a reading of
zero.
Transistor Q 1 may be replaced by any
on e of a number of high-beta NPN typ es.
Try .what yo u have around and see if it
wor ks. R2 may be used to balan ce slight
differences if another transistor is used.
Calibrate by testing a precision IOn resi s­
tor (O~10Q scale) and setting the calibration
control for ,a full-scale reading. The
other scale should foll ow automatic ally.
87